_Habitat and numbers._--In Veracruz, Dalquest obtained the southern pygmy mouse in stands of tall grass (_Spartina?_) in sandy loam soil bordering, and in, dense vegetation; Davis (1944:394) found the species living in dense stands of grasses and seemingly utilizing underground burrows. Near Chilpancingo, Guerrero, rocky situations seemed to be the preferred habitat. Davis (_loc. cit._) believed that the species has a wide tolerance to kinds of habitats. In Morelos, Davis and Russell (1954:75) found these mice to be abundant along rock fences separating cultivated fields, and in arid lowlands. In Colima, Hooper (1955b:13) obtained specimens from an open thorn forest in sparse grass and rocky hillside bounding a stream and in litter below shrubs on the floor of a nut-palm forest; in Michoacan, these mice were taken in cane grass, shrubs, and mesquite near an irrigation ditch. From Guatemala, Goodwin (1934:39, 40) records specimens from Sacapulas, a hot, dry, sandy area where cactus and sparse grasses are present, and from La Primavera, on the edges of pine-oak-alder forests. Felten (1958:137) has taken _musculus_ from bushy areas in El Salvadore. In 1955, I obtained the southern pygmy mouse 6 mi. SW Izucar de Matemores, Puebla, along a stream in heavy grass bordered by cypress, willow, fig, bamboo, and in rocky grazed area near sugar cane fields. The southern pygmy mouse seems to be locally abundant in certain parts of its geographic range, and in other parts, scarce. For example, Dalquest (_in. litt._) recorded the pygmy mouse as common at a place 2 km. N Paraje Nuevo, 1700 feet, Veracruz, where, by means of 50 traps, he took 14 of these mice in one night. The species was scarcer, although the habitat seemed suitable, 3 km. N Presidio, 1500 feet, Veracruz, where he caught only two pygmy mice in several days of trapping. Six miles southwest of Izucar de Matemores, the pygmy mouse was the most common rodent. I have trapped for it in Oaxaca and Veracruz in habitats that seemed almost identical to those mentioned by Dalquest, and also that at Izucar de Matemores, Puebla, with almost no success. The reason for the seeming disparity in numbers at different localities having nearly the same kind of habitat is unknown to me and bears further investigation. _Behavior._--Little is recorded concerning the behavior of this species. David and Russell (_op. cit._:76) found that of small mammals _B. musculus_ was the first to appear at night. I caught mice of this species by hand in the afternoon in Puebla. They seemed to be active from noon until dark. Albert Alcorn wrote in his field notes that specimens were taken near noon at a place 9 mi. NNW Esteli, Nicaragua. My impression is that _musculus_ is diurnal to crepuscular.